Whenever I think of the Sun’s influence on the Earth’s climate I think of the difference between relative and absolute numbers, and how they can provide completely different views of a situation.
Very often articles about the Sun’s effects on the Earth’s climate state that it must be small because variations in TSI (Total Solar Irradiance) takes place on the 0.1% level over the approximately 11-year solar cycle (measured over the last three cycles). This is very small so how can it have any influence, many argue, pointing out that considered this way the Sun is a very constant star.
Indeed, we call the most familiar measure of its output the solar constant – the electromagnetic flux density at the Sun-Earth distance. It is 1,361 Watts per sq metre. So 0.1% of this is 1.3 Watts per sq metre. When looked at in absolute terms this is not a little amount of energy. In fact it’s an enormous amount of energy, as has been pointed out by reputable sources in the past.
The solar constant isn’t constant. It varies from day to day due to the occurrence of sunspots and faculae, and throughout the year due to the changing distance to the Sun of the Earth’s orbit. In January it is 1,412 Watts per sq metre and in July 1,321 Watts per sq metre, a variation of 7%. Longer term variations than the solar cycle in TSI there must be, but estimates are very uncertain.
The climate forcing factor for CO2 is currently about 1.5 Watts per sq metre. The major difference between the two is that CO2 forcing is always positive and increasing whereas solar TSI forcing is cyclical. However, for the 3 – 4 years when the solar cycle is increasing in activity the solar climate forcing factor is comparable to the CO2 forcing factor, meaning that for a third of the time the positive forcing factor of CO2 is enhanced. It’s curious that this effect is not visible in the global temperature datasets.
I am also struck by the difference in outlook between climatologists who consider the sun, and solar physicists who consider climatology. Individual opinion may vary but broadly speaking solar physicists are more likely to see the sun as having a larger influence. Both sides agree however that solar influences can dominate over greenhouse gas effects in regional situations for perhaps a few decades. In some people’s considerations those regions can be quite widespread. Overall you don’t have to look far to see the range of opinion that exists concerning the Sun’s influence on climate.
So when and how much is the Sun’s influence? Before the industrial era climate variations were obviously natural, so many consider climate changes being caused by solar variations combined with volcanic effects.
The IPCC says that the cutoff between natural and anthropogenic dominance is between 1960-80. It believes that after 1960 – 80 (a 0.4 deg C increase between 1980 – 2000) climate change is overwhelmingly due to CO2 and cannot be explained without it. Everything before that (a 0.5 deg C rise from 1880 – 1940) was down to the Sun and volcanoes. It has always surprised me the ease with which some scientists accepted the view that the sun had a dominant effect on the rise of global temperatures of 0.5 deg C in the first part of the 20th century, and no effect in the rise of 0.4 deg C in the later part of the century, indicating that the dominance of greenhouse gasses coincided with a cessation of solar effects!
The sitting of the 75-year long 17th century Maunder Minimum inside the longer Little Ice Age provides some evidence that changes in solar activity does have a climatic effect. Some scientists have tried to explain the Little Ice Age as due to a combination of low solar TSI and volcanic effects, though not very successfully in my opinion. One study (Shapiro, Astronomy and Astrophysics 2011) suggests a much larger increase in TSI since the Maunder Minimum, 6 Watts per sq Metre.
Many consider that TSI variations suggest that the solar influence on climate is at the 0.1 deg C level per decade. If this is applicable to the recent warming spell (the one that can’t be explained without anthropogenic effects remember) then it reduces its anthropogenic component. This is reduced even more by the possibility that natural variations in stratospheric water vapour might be responsible for a third of the warming between 1980 -2000. These two effects, leaving aside other decadal variations such as the ocean cycles, thus account for over half of the recent warming, something that is at odds with the IPCC’s assessment.
But there is a different strand of opinion. Some studies have suggested that the Sun’s changes in brightness over the past thousand years have had little effect on the Earth’s climate, although it leaves the door open to other routes of influence. One coauthor said: “Our results imply that, over the past century, climate change due to human influences must far outweigh the effects of changes in the Sun’s brightness. If this is so then the climb out of the Little Ice Age is difficult to explain.”
Given these uncertainties it is difficult to access the Sun’s influence on future climate change given the well-publicised downturn in its activity seen in the past decade.
The outcome of a recent paper tackling this question is rather obvious. Simple climate models projected, in this case, forward to 2090, will be dominated by greenhouse gas forcing which will be much larger than any diminution cause by the sun based on its TSI variations. No surprise there.
But will the Earth’s global temperature start to rise again after its 10-15 year standstill? Is the recent past a deviation from the long-term 0.2 deg C per decade average the IPCC predict? Will warming resume?
I have a feeling that the real world data as well as questions, parameters and opinions may change quite a bit long before 2090.